Shortly after the age of forty, many people experience a decrease in their ability to accommodate or properly focus their vision on objects located at arbitrary distances. Most commonly, people lose the ability to see near objects within reading distance. This condition is technically known as presbyopia (from the Greek "old eye"), and results from changes in the physiology of the human eye. Presbyopia coupled with the rising average age of the general population has increased the demand for bifocal ophthalmic lenses capable of correcting failing vision.
Conventional ophthalmic bifocal lenses known to the prior art can be classified into two basic categories. The first is the segmented type lens which is composed of distinct parts having differing radii of curvature and/or differing indices of refraction. Thus, the lens has different refractive powers and the user may focus on near objects by viewing through one of the segments or focus on far objects by viewing through the remaining portion of the lens. The second type of conventional bifocal lens consists of a classical refractive lens to which a diffractive element or pattern has been applied. The diffractive element in combination with the refractive power of the lens produces one focal length for near objects, while the undiffracted light from far objects focuses according to the refractive power of the lens. This results in two images being simultaneously present on the user's retina. The user's eye/brain system is then capable of discerning between these two images depending upon whether a near or a far object is being viewed. Examples of the two types of conventional ophthalmic bifocal lenses are described and illustrated in U.S. Pat. No. 4,302,081, issued to G. F. Tsuetaki, and U.S. Pat.No. 4,642,112 issued to M. H. Freeman.
The primary disadvantage of both the segmented lens and the diffractive element lens lies in the difficulty of fabrication which requires special machining or structuring on the surface of the lens. In addition, the segmented lens has problems associated with alignment, particularly when used as a contact lens. Also, the diffracted image resulting from the diffractive element lens can have chromatic aberration and imaging noise due to scattering and higher order diffraction. Excellent discussions of the problems encountered with the prior art bifocal lenses can be found in W. N. Charman, "Diffractive Bifocal Contact Lenses", Contax, 11, (May 1986); and S. St. Cyr, "The Holographic Bifocal Contact Lens", Holosphere, Vol. 15, No. 5, 14, (Fall 1988).
Consequently, a need exists for improvements in bifocal ophthalmic lenses which will result in improved performance and decreased difficulty in manufacture.